2.0 Analysis 2.1 Warnings and Alerts The EQPT COOLING OVRD and the WINDOW HEAT L FWD INOP warnings appeared in the cockpit at 1503, approximately 1minute and 15seconds after the start of the fire and 10to 15minutes after the onset of an odour in the back of the aircraft. The crew indicated that the alerts appeared in close succession to one another after the use of the fire extinguisher, with the WINDOWHEATLFWDINOP appearing first. It is possible that the cool Halon1211 streaming agent and the dynamic pressure of the gas hitting the wires and terminal block (especially if the terminal insert was loose due to the burning of the polysulphide filler/adhesive) could have caused a thermal shock causing the WHCU to detect a current interruption and turn on the warning. The fault detection functions built into the WHCU did not pick up the overheating at the J5connector early on in the occurrence and gave no system protection or advance warning of a problem until after the fire began, nor is the system designed to do so. The fire extinguished when the electrical power to the system was interrupted and the polysulphide filler/adhesive was consumed. The EQPT COOLING OVRD light appeared about 14seconds after the crew's first use of the fire extinguisher. The use of the extinguisher likely produced enough condensed vapour (steam) and decomposition by-products, to cause the air that was being drawn from behind the flight deck forward instrument panel to activate the RSD, thereby turning on the light. 2.2 Manufacturing Issues The soldering of the braid conductor from the anti-ice and anti-fog elements to the window terminal brass insert is a hand-operated process with no means of mechanically holding the braid before or after the soldering process. Good soldering practices for electrical connections require a physical connection such as clenching, followed by soldering to fill the spaces and provide low resistance. A clenched and soldered joint results in a stronger connection and reduces the likelihood of a cold-soldered joint. Solder has a relatively low melting point. Where high-current windshield de-icing circuits depend on solder alone for the physical security of the electrical connection, there is a risk of electrical arcing if the terminal screw connection overheats and the solder melts. 2.3 Window Failure Due to the extent of the burn damage, it could not be determined whether the overheating of the J5terminal occurred as a result of a manufacturing defect, such as a poor solder connection, or because of resistive heating caused by a loose terminal connector screw. Debris was noted on the J5 connector and brass block mating surfaces, as well as on the threads inside the terminal block. The debris could be indicative of heated foreign material entering the contact area due to a gap provided by a loose connection. A tight connection would probably not allow the ingestion of foreign material during heating, unless the material was already present during assembly. Boeing issued an All-Operator Message (7240-93-1844), indicating that insufficient torque on the connector screw may lead to a loosening of the screw in service. As a number of other terminal screws on the aircraft were found undertorqued, the possibility of the J5connector also being initially undertorqued is strong. As the connector loosens in service, the Oring protecting the mating surfaces, normally in compression, relaxes, effectively separating the mating surfaces and causing the electrical power to be transmitted through the terminal screw. Once large amounts of electrical current are directed through the terminal screw, overheating of the connection can occur.